1. Field of the Invention
The present invention relates to luminescent devices which have a luminescent layer containing a luminescent material and which directly convert applied electrical energy into optical energy. In particular, the present invention relates to a thin, light and solid luminescent device with a large luminescent area and high resolution, enabling high-speed operation. Such a luminescent device is quite different from conventional incandescent lamps, fluorescent lamps, and light emitting diodes (LEDs), and can be used for electroluminescence panels which are expected as devices satisfying advanced needs in various industrial fields.
2. Description of the Related Art
Pope et al. first discovered an electroluminescence (EL) of an organic material, that is, single-crystal anthracene, in 1963 (J. Chem. Phys., 38, 2042 (1963)). Helfinch and Schneider succeeded with observation of relatively strong EL in an injection EL material containing a solution system having a high injection efficiency in 1965 (Phys. Rev. Lett., 14, 229 (1965)). Many studies of organic luminescent materials containing conjugated organic hosts and conjugated organic activators having condensed benzene rings have been disclosed in U.S. Pat. Nos. 3,172,862, 3,173,050, and 3,710,167; J. Chem. Phys., 44, 2902 (1966); J. Chem. Phys., 58, 1542 (1973); and Chem. Phys. Lett., 36, 345 (1975). Examples of disclosed organic hosts include naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzpyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene. Examples of disclosed activators include anthracene, tetracene and pentacene. Since these organic luminescent materials are provided as single layers having a thickness of more than 1 xcexcm, a high electric field is required for luminescence. Under these circumferences, thin film devices formed by a vacuum deposition process have been proposed (for example, xe2x80x9cThin Solid Filmsxe2x80x9d, p. 94 (1982); Polymer, 24, 748 (1983); and J. Appl. Phys., 25, L773 (1986)). Although the thin film devices are effective for reducing the driving voltage, their luminance is far from a level sufficient for practical use.
In recent years, Tang et al. have developed an EL device having a high luminance for a low driving voltage (Appl. Phys. Lett., 51, 913 (1987) and U.S. Pat. No. 4,356,429). The EL device is fabricated by depositing two significantly thin layers, that is, a charge transport layer and a luminescent layer, between the anode and the cathode by a vacuum deposition process. Such layered organic EL devices are disclosed in, for example, Japanese Patent Laid-Opens Nos. 59-194393, 3-264692, and 3-163188, U.S. Pat. Nos. 4,539,507 and 4,720,432, and Appl. Phys. Lett., 55, 1467 (1989).
Also, an EL device of a triple-layered structure having independently a carrier transport function and a luminescent ability was disclosed in Jpn. J. Apply. Phys., 27, L269 and L713 (1988). Since the carrier transportability is improved in such an EL device, the versatility of possible dyes in the luminescent layer is considerably increased. Further, the device configuration suggests feasibility of improved luminescence by effectively trapping holes and electrons (or excimers) in the central luminescent layer.
Layered organic EL devices are generally formed by vacuum deposition processes. EL devices having considerable luminance are also formed by casting processes (as described in, for example, Extended Abstracts (The 50th Autumn Meeting (1989), p. 1006 and The 51st Autumn Meeting (1990), p. 1041; The Japan Society of Applied Physics). Considerably high luminance is also achieved by a single-layered mixture-type EL device, in which the layer is formed by immersion-coating a solution containing polyvinyl carbazole as a hole transport compound, an oxadiazole derivative as an electron transport compound and coumarin-6 as a luminescent material (as described in Extended Abstracts (The 38th Spring Meeting (1991), p. 1086; The Japan Society of Applied Physics and Related Societies).
As described above, the organic EL devices have been significantly improved and have suggested feasibility of a wide variety of applications; however, these EL devices have some problems for practical use, for example, insufficient luminance, a change in luminance during use for a long period, and deterioration by atmospheric gas containing oxygen and humidity. Further, it is hard to say that the EL devices sufficiently satisfy needs of diverse wavelengths of luminescent light for precisely determining luminescent hues of blue, green and red colors in full-color displays etc.
It is an object of the present invention to provide a luminescent device having high output luminance for solving the above-mentioned problems.
It is another object of the present invention to provide a luminescent device having excellent durability.
It is a further object of the present invention to provide a luminescent device easily produced at relatively low production costs.
A luminescent device in accordance with the present invention includes an anode, a cathode, and an organic layer containing at least one organic compound provided therebetween, wherein the work function of the anode Wf(anode) and the Fermi level of the organic layer (Ef(anode)) upon contact with the anode satisfy the following equation (I):
Ef(anode)xe2x88x920.2xe2x89xa6Wf(anode)xe2x89xa6Ef(anode)+0.2[eV]xe2x80x83xe2x80x83(I) 
Preferably, the work function of the cathode (Wf(cathode)) and the Fermi level of the organic layer (Ef(cathode)) upon contact with the cathode satisfy the following equation (II):
Wf(cathode)xe2x89xa6Ef(cathode)[eV]xe2x80x83xe2x80x83(II) 
The luminescent device satisfying equation (I) or (II) can produce light with significantly high luminance by a low applied voltage and has excellent durability.
A luminescent device with a large area can be easily formed by a vacuum deposition or casting process with low production costs.